1. Field of the Invention
This invention relates generally to the production of single-wall nanotubes; in particular, to gas-phase nucleation and growth of single-wall carbon nanotubes from high pressure CO.
2. Description of Related Art
Fullerenes are closed-cage molecules composed entirely of sp2-hybridized carbons, arranged in hexagons and pentagons. Fullerenes (e.g., C60) were first identified as closed spheroidal cages produced by condensation from vaporized carbon.
Fullerene tubes are produced in carbon deposits on the cathode in carbon arc methods of producing spheroidal fullerenes from vaporized carbon. Ebbesen et al. (Ebbesen I), xe2x80x9cLarge-Scale Synthesis Nanotubes,xe2x80x9d Nature, Vol. 358, p. 220 (Jul. 16, 1992) and Ebbesen et al., (Ebbesen II), xe2x80x9cCarbon Nanotubes,xe2x80x9d Annual Review of Materials Science, Vol. 24, p. 235 (1994). Such tubes are referred to herein as carbon nanotubes. Many of the carbon nanotubes made by these processes were multi-wall nanotubes, i.e., the carbon nanotubes resembled concentric cylinders. Carbon nanotubes having up to seven walls have been described in the prior art. Ebbesen II; Iijima et al., xe2x80x9cHelical Microtubules Of Graphitic Carbon,xe2x80x9d Nature, Vol. 354, p. 56 (Nov. 7, 1991).
Single-wall carbon nanotubes have been made in a DC arc discharge apparatus of the type used in fullerene production by simultaneously evaporating carbon and a small percentage of Group VIII transition metal from the anode of the arc discharge apparatus. See Iijima et al., xe2x80x9cSingle-Shell Carbon Nanotubes of 1 nm Diameter,xe2x80x9d Nature, Vol. 363, p. 603 (1993); Bethune et al., xe2x80x9cCobalt Catalyzed Growth of Carbon Nanotubes with Single Atomic Layer Walls,xe2x80x9d Nature, Vol. 363, p. 605 (1993); Ajayan et al., xe2x80x9cGrowth Morphologies During Cobalt Catalyzed Single-Shell Carbon Nanotube Synthesis,xe2x80x9d Chem. Phys. Lett., Vol. 215, p. 509 (1993); Zhou et al., xe2x80x9cSingle-Walled Carbon Nanotubes Growing Radially From YC2 Particles,xe2x80x9d Appl. Phys. Lett., Vol. 65, p. 1593 (1994); Seraphin et al., xe2x80x9cSingle-Walled Tubes and Encapsulation of Nanocrystals Into Carbon Clusters,xe2x80x9d Electrochem. Soc., Vol. 142, p. 290 (1995); Saito et al., xe2x80x9cCarbon Nanocapsules Encaging Metals and Carbides,xe2x80x9d J. Phys. Chem. Solids, Vol. 54, p. 1849 (1993); Saito et al., xe2x80x9cExtrusion of Single-Wall Carbon Nanotubes Via Formation of Small Particles Condensed Near an Evaporation Source,xe2x80x9d Chem. Phys. Lett., Vol. 236, p. 419 (1995). It is also known that the use of mixtures of such transition metals can significantly enhance the yield of single-wall carbon nanotubes in the arc discharge apparatus. See Lambert et al., xe2x80x9cImproving Conditions Toward Isolating Single-Shell Carbon Nanotubes,xe2x80x9d Chem. Phys. Lett., Vol. 226, p. 364 (1994). High quality single-wall carbon nanotubes have also been generated by arc evaporation of a graphite rod doped with Y and Ni. See C. Journet et al., Nature 388 (1997) 756, hereby incorporated by reference in its entirety. These techniques allow production of only gram quantities of single-wall carbon nanotubes at low yield of nanotubes and the tubes exhibit significant variations in structure and size between individual tubes in the mixture.
An improved method of producing single-wall nanotubes is described in U.S. patent application Ser. No. 08/687,665, entitled xe2x80x9cRopes of Single-Walled Carbon Nanotubesxe2x80x9d incorporated herein by reference in its entirety. This method uses, inter alia, laser vaporization of a graphite substrate doped with transition metal atoms, preferably nickel, cobalt, or a mixture thereof, to produce single-wall carbon nanotubes in yields of at least 50% of the condensed carbon. See A. Thess et al., Science 273 (1996) 483; T. Guo. P. Nikolaev, A. Thess, D. T. Colbert, R. E. Smalley, Chem. Phys. Lett., 243, 49-54 (1995), both incorporated herein by reference. The single-wall nanotubes produced by this method tend to be formed in clusters, termed xe2x80x9cropes,xe2x80x9d of 10 to 1000 single-wall carbon nanotubes in parallel alignment, held together by van der Waals forces in a closely packed triangular lattice. Nanotubes produced by this method vary in structure, although one structure tends to predominate. These high quality samples have for the first time enabled experimental confirmation of the structurally dependent properties predicted for carbon nanotubes. See J. W. G. Wildoer, L. C. Venema, A. G. Rinzler, R. E. Smalley, C. Dekker, Nature, 391 (1998) 59; T. W. Odom, J. L. Huang, P. Kim, C. M. Lieber, Nature, 391 (1998) 62. Although the laser vaporization process produces improved single-wall nanotube preparations, the product is still heterogeneous, and the nanotubes are too tangled for many potential uses of these materials. In addition, the vaporization of carbon is a high energy process and is inherently costly.
Another known way to synthesize nanotubes is by catalytic decomposition of a carbon-containing gas by nanometer-scale metal particles supported on a substrate. The carbon feedstock molecules decompose on the particle surface, and the resulting carbon atoms then diffuse through the particle and precipitate as a part of nanotube from one side of the particle. This procedure typically produces imperfect multi-walled nanotubes in high yield. See C. E. Snyder et al., Int. Pat. WO 9/07163 (1989), hereby incorporated by reference in its entirety.
Yet another method for production of single-wall carbon nanotubes involves the disproportionation of CO to form single-wall carbon nanotubes +CO2 on alumina-supported transition metal particles such as Mo, Mo/Fe, and Ni/Co. See Dai, H. J. et al., xe2x80x9cSingle-Wall Nanotubes Produced by Metal-Catalyzed Disproportionation of Carbon Monoxide,xe2x80x9d Chem. Phys. Lett., 1996. 260(3-4): p. 471-475. In this process the transition metal particles on the alumina support that were large enough to produce multi-walled nanotubes were preferentially deactivated by formation of a graphitic overcoating, leaving the smaller metal particles to catalyze the growth of single-wall carbon nanotubes. Good quality single-wall carbon nanotubes can be grown from alumina-supported catalysts even with hydrocarbon feed stocks such as ethylene, provided the multi-walled nanotube production is suppressed by a pretreatment process. See Hafner, H. F. et al., xe2x80x9cCatalytic Growth of Single-Wall Carbon Nanotubes From Metal Particles,xe2x80x9d Chem. Phys. Lett., 1998. 296(1-2): p. 195-202; and U.S. Provisional Patent Application No. 60/101,093, entitled xe2x80x9cCatalytic Growth of Single Wall Carbon Nanotubes from Metal Particles,xe2x80x9d and International Application No. PCT/US99/21367, hereby incorporated by reference in their entirety. These methods use cheap feed stocks in a moderate temperature process. Their yield is intrinsically limited due to rapid surrounding of the catalyst particles and the alumina particle that supports them by a dense tangle of single-wall carbon nanotubes. This tangle acts as a barrier to diffusion of the feedstock gas to the catalyst surface, inhibiting further nanotube growth. Removal of the underlying alumina support from the nanotubes that form around it will be an expensive process step.
Hollow carbon fibers that resemble multi-walled carbon nanotubes have been produced from entirely gas phase precursors for several decades. See Dresselhaus, M. S., G. Dresselhaus, and P. C. Ecklund, Science of Fullerenes and Carbon Nanotubes, 1996, San Diego: Academic Press, 985. Endo first used ferrocene and benzene vapors traveling through a quartz tube in an Ar+H2 carrier gas at about 1000xc2x0 C. to make carbon nanotubes (imperfect multi-walled carbon nanotubes) overcoated in a largely amorphous carbon. See Endo, M., xe2x80x9cGrow carbon fibers in the vapor phase,xe2x80x9d Chemtech, 1988: p. 568-576. Tibbetts has used ferrocene and iron pentacarbonyl to produce similar hollow carbon fibers from methane/hydrogen mixtures at 1000xc2x0 C., a process that he finds is benefited by the addition of sulfur in the form of H2S. See Tibbetts, G. G., xe2x80x9cVapor-Grown Carbon Fibers: Status and Prospects. Carbon,xe2x80x9d 1989. 27(5): p. 745-747. In some of Endo""s early experiments it is clear that small amounts of single-wall carbon nanotubes were produced as well. But until recently no means has been found to adapt these gas phase methods to produce primarily single-wall carbon nanotubes.
Very recently it has been found that control of the ferrocene/benzene partial pressures and addition of thiophene as a catalyst promoter in the all gas-phase process can produce single-wall carbon nanotubes. See Sen, R. et al., xe2x80x9cCarbon Nanotubes By the Metallocene Route,xe2x80x9d Chem. Phys. Lett., 1997 267(3-4): p. 276-280; Cheng, H. M. et al., xe2x80x9cLarge-Scale and Low-Cost Syntheses of Single-Wall Carbon Nanotubes By the Catalytic Pyrolysis of Hydrocarbons,xe2x80x9d Appl. Phys. Lett., 1998. 72(25): p. 3282-3284; Dresselhaus, M. S., xe2x80x9cCarbon Nanotubesxe2x80x94Introduction,xe2x80x9d Journal of Materials Research, 1998. 13(9): p. 2355-2356. However, all these methods involving hydrocarbon feed stocks suffer unavoidably from the simultaneous production of multi-walled carbon nanotubes, amorphous carbon, and other products of hydrocarbon pyrolysis under the high temperature growth conditions necessary to produce high quality single-wall carbon nanotubes.
Therefore, there remains a need for improved methods of producing single-wall nanotubes of greater purity and homogeneity.
The present invention provides a method and apparatus for the efficient, industrial scale production of single-wall carbon nanotubes (SWNTs) from all gaseous reactants and which product is substantially free of solid contaminants or by-products (e.g., amorphous carbon deposits). This process is based on the use of high pressure CO as the carbon source and an appropriate gaseous transition metal catalyst precursor.
The present invention provides a method for producing single wall carbon nanotube products comprising the steps of: (a) providing a high pressure CO gas stream; (b) providing a gaseous catalyst precursor stream comprising a gaseous catalyst precursor that is capable of supplying atoms of a transition metal selected from Group VI, Group VIII or mixture thereof, said gaseous catalyst precursor stream being provided at a temperature below the decomposition temperature of said catalyst precursor; (c) heating said high pressure CO gas stream to a temperature that is (i) above the decomposition temperature of said catalyst precursor and (ii) above the minimum Boudouard reaction initiation temperature, to form a heated CO gas stream; and (d) mixing said heated CO gas stream with said gaseous catalyst precursor stream in a mixing zone to rapidly heat said catalyst precursor to a temperature that is (i) above the decomposition temperature of said catalyst precursor, (ii) sufficient to promote the rapid formation of catalyst metal atom clusters and (iii) sufficient to promote the initiation and growth of single-wall nanotubes by the Boudouard reaction, to form a suspension of single wall carbon nanotube products in the resulting gaseous stream.
The present invention also provides an apparatus for producing single wall carbon nanotube products comprising: (a) a high pressure reaction vessel comprising in serial communication a reactant introduction in zone, a reactant mixing zone, a growth and annealing zone and a product recovery zone; (b) a first reactant supply conduit for supplying a heated high pressure CO gas to said introduction zone; (c) a second reactant supply conduit for supplying a gaseous catalyst precursor to said information zone; (d) mixing means for rapidly and intimately mixing the gas flows from said first and second reactant supply conduits as said flows enter said mixing zone; (e) heating means for maintaining said growth and annealing zone at an elevated temperature; and (f) gas/solids separation means positioned in said product recovery zone to remove solid single wall carbon nanotube products from the gas flows exiting said growth and annealing zone.
The present invention further provides a composition of matter comprising single-wall carbon nanotubes having a tube diameter in the range of 0.6 nm to 0.8 nm.
The present invention further provides a composition of matter comprising (5,5) single-wall carbon nanotubes.
The process involves supplying high pressure (e.g., 30 atmospheres) CO that has been preheated (e.g., to about 1000xc2x0 C.) and a catalyst precursor gas (e.g., Fe(CO)5) in CO that is kept below the catalyst precursor decomposition temperature to a mixing zone. In this mixing zone, the catalyst precursor is rapidly heated to a temperature that results in (1) precursor decomposition, (2) formation of active catalyst metal atom clusters of the appropriate size, and (3) favorable growth of SWNTs on the catalyst clusters. Preferably a catalyst cluster nucleation agency is employed to enable rapid reaction of the catalyst precursor gas to form many small, active catalyst particles instead of a few large, inactive ones. Such nucleation agencies can include auxiliary metal precursors that cluster more rapidly than the primary catalyst, or through provision of additional energy inputs (e.g., from a pulsed or CW laser) directed precisely at the region where cluster formation is desired. Under these conditions SWNTs nucleate and grow according to the Boudouard reaction. The SWNTs thus formed may be recovered directly or passed through a growth and annealing zone maintained at an elevated temperature (e.g., 1000xc2x0 C.) in which tubes may continue to grow and coalesce into ropes.
The SWNT products can be separated from the gaseous stream and recovered. The gaseous stream, which is primarily unreacted CO can be recovered and recycled. The resulting SWNT products are substantially pure (99%) and can be used without complicated separation and purification steps. The process of this invention also provides the ability to reproducibly control the diameter of SWNT products produced. This process also provides the first SWNT process that can produce a product that is substantially made up of small diameter nanotubes (e.g., (5,5) tubes).